Diagnosis
The plastic locking latches on the tool's battery port are worn, or the corresponding grooves on the battery pack are packed with dirt and debris.
How to Fix It
Inspect the battery release latches on the tool. Press them to ensure they spring back. Then, clean the battery's contact grooves and the tool's guide rails with a dry toothbrush and compressed air. Do not use lubricant.
If That Didn't Work
The spring mechanism for the latches inside the tool housing is broken or dislodged. This requires disassembly of the main housing to access the latch system. The plastic latches themselves can be replaced if broken.
Heads Up
A loose battery connection causes arcing at the contacts. This creates heat, melts plastic, and permanently damages both the tool and battery. Stop using it until the connection is solid.